Our research focuses on the active centers of iron proteins that are involved in O2 transport or activation, electron transfer, and related functions. The goal is to elucidate the electronic state and the dynamical properties of the iron, to study interactions between paramagnetic centers, and to relate the physical parameters of these proteins to their function. Specifically, we are interested in the microbial heme enzymes cytochrome P450CAM from Pseudomonas putida, chloroperoxidase from Caldariomyces fumago, catalase from Micrococcus luteus, cytochrome c oxidase/nitrite reductase from Pseudomonas aeruginosa, and the iron-sulfur protein amido transferase from Bacillus subtilis. Experiments are carried out in close cooperation with the biochemists who developed these systems and elaborate them further. By combining 57Fe Mossbauer spectroscopy, our main tool, with EPR we hope to settle the controversial questions concerning (1) compound I of chloroperoxidase and catalase, (2) the activated oxy intermediate of cytochrome P450CAM, and (3) the postulated triplet state of oxy hemoglobin and related O2 aducts. These comparative studies should lead to a better understanding of the different reactivities of hemeproteins towards oxygen.